As public safety agencies and other organizations evaluate their two-way radio needs for the future, a number of possible solutions are available, including various digital radio technologies.
For instance, the Association of Public-Safety Communications Officials (APCO) Project 25 (P25) (or APCO-25) represents one effort to set standards for digital two-way radio technology. In general, P25 refers to a suite of narrowband digital Land Mobile Radio (LMR) communication standards for digital radio communications, equipment and systems. P25 standards are produced through the joint efforts of the Association of Public Safety Communications Officials International (APCO), the National Association of State Technology Directors (NASTD), selected Federal Agencies and the National Communications System (NCS), and standardized under the Telecommunications Industry Association (TIA). Further details regarding the P25 standards can be obtained from the Telecommunications Industry Association, 2500 Wilson Boulevard, Suite 300 Arlington, Va. 22201.
P25 standards specify a Common Air Interface (CAI) that defines the type and content of signals transmitted by P25-compliant radios. P25-compliant radios can communicate directly with each other in “talk around” mode without any intervening equipment between two radios, or in conventional mode where a requesting radio chooses the channel to talk on and two radios communicate through a repeater or base station without trunking. In addition, two radios may communicate in a trunked mode where infrastructure equipment dynamically assigns the requesting radio a channel to talk on and traffic is automatically assigned to one or more voice channels on a repeater or base station.
P25-compliant technology is being deployed in several phases commonly referred to as Phase 1 and Phase 2. Phase 2 is currently under development to improve spectrum utilization. Among other changes to the Phase 1 standard, the Phase 2 standard proposes significant changes to the CAI. One of the major changes is the migration from a Frequency Division Multiple Access (FDMA)-based channel access scheme to a Time Division Multiple Access (TDMA)-based channel access scheme. As such, synchronization between the mobile radio and the base station is of high importance, and therefore methods for detecting synchronization problems are needed. In addition, in the Project 25 Phase 2 standard, the air interface payload is scrambled, and therefore methods for detecting scrambling problems are needed.
In any wireless communication system, radio signals are subject to wide power level variations over time due to shadowing, fading, change in distance between a mobile radio and a base station, and the like, and thus exhibit signal quality variations with respect to the communication links used for communicating between them. As used herein, any condition where the radio signal strength weakens to the point where radio communications are affected can be referred to as “fading.” Fading can be due to short term variations in radio signals (e.g., Raleigh fading), or due to an increase in distance between the transmitting radio and the receiving radio.
As such, it is important to provide a mechanism for infrastructure equipment to determine when a call (or communication session) has ended so that communication resources assigned to that call can used by other mobile stations. From the perspective of the mobile radio, radio interfaces are challenging to establish and maintain. When a trunked radio voice connection is interrupted, the mobile radio needs to decide if the call is still viable. Phase 2 trunked voice radio connections can be compromised in a number of ways, including improper synchronization, incorrect scrambling, and signal interruptions. The signal interruptions can interfere with the mobile radio's synchronization and data extraction. Therefore, it is also important to provide a mechanism for mobile radios to determine when fading conditions are occurring, and to provide mechanisms for dealing with such fading conditions.
In some wireless communication systems, when a mobile radio completes a transmission it can transmit an explicit termination instruction (sometimes called an end-of-call message) to infrastructure equipment. In response the infrastructure equipment terminates the call, and the communication channel and other resources may then be made available for other calls. In some cases a mobile station does not intend to end its call, but moves out of range of the base station that it is communicating with. Because the mobile station does not intend to end the call, an end-of-call message is not transmitted, and hence the infrastructure equipment (e.g., base station) does not receive an explicit end-of-call message from that mobile radio. In other cases, the mobile station might transmit an end-of-call indication, but it may not be received by the infrastructure equipment or might, for example, contain too many errors to be decoded correctly.
When a base station completes a voice transmission in a trunked system by sending an end-of-call message, the mobile radio switches to the control channel to be available for other calls.
However, when fade conditions occur (e.g., when a drop in the radio frequency (RF) power level truncates the end of the call), the mobile radio should nevertheless be able to detect and confirm that the call has ended. The mobile radio needs a mechanism to switch to the control channel without receiving an explicit instruction from the base station.
At the same time, because fading happens regularly in a wireless environment, it is desirable for the mobile radio not to prematurely drop a call just because a temporary fade condition has been detected. In the event a call is prematurely dropped due to fading and the mobile station is still within the coverage area of the base station, the mobile radio should also be able to resume the call as soon as possible. The mechanism employed by the mobile radio to switch to the control channel (without explicit instruction from the base station) not only has to be reliable, but it also has to be relatively insensitive to fringe RF conditions. An operational goal is to minimize the probability that a viable call on the fringe of coverage is terminated while still truncating non-viable calls reasonably quickly to reduce the probability of missing subsequent calls.
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The apparatus and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein.